1. Field of the Invention
The present invention relates to acknowledgement signals, and more particularly, acknowledgment signals in mobile communication.
2. Background of the Related Art
Mobile radio channels are often characterized by the unpredictability of the channel due to, inter alia, Rayleigh fading and long term shadow fading. The channel quality may degrade as a result of several factors such as, for example, co-channel interference, adjacent channel interference, propagation path loss, and multi-path propagation (i.e., rayleigh fading). Transmission errors typically occur in bursts when fading causes the signal level to go below the noise or interference level. Therefore, explicit measures often need to be taken to maintain an acceptable level of quality of the transmission over a radio channel.
The quality of the transmission over a radio channel connection may be measured by the reliability with which the receiver receives the transmitted data. This channel reliability may, for example, be defined in terms of the bit-error-rate (BER) experienced at the receiver.
Specifically, forward error correction (FEC) and automatic repeat request (ARQ) are two well-known error control techniques commonly used for noisy and fading channels. In a system that uses FEC for error control, for example, the transmitter encodes the data using a given redundancy code, while the receiver, which has been informed of the code used, decodes the data at the receiving end. Many such systems using conventional block or convolutional codes have been explored and/or employed.
In a system that uses ARQ, the receiver returns (i.e., transmits back to the transmitter) an acknowledgement which indicates whether the given transmitted packet was received free of errors (in which case an acknowledgement signal, or “ACK” is sent), or whether it was received erroneously (in which case a negative acknowledgement signal, or “NACK” is sent). If the packet was not received error-free (i.e., if the transmitter receives back a “NACK” signal), the transmitter then retransmits the same packet again, anticipating that the packet will be successfully received on this (or else on a further, subsequent) transmission.
Transmission of multimedia applications such as high quality audio, images and real-time video, for example, require very low bit-error-rates, typically 10−6 or less. Obtaining such low BERs in wireless environments is challenging, even in the presence of very low rate forward error correction codes.
ARQ techniques, however, provide very reliable communication, albeit at the expense of variable and sometimes large delays. But hybrid ARQ schemes, in which both FEC and ARQ techniques are employed simultaneously, are particularly attractive because they combine the fixed delay error correction capability of FEC techniques with the low BER of basic ARQ schemes.
Though several ARQ schemes are utilized, the ACK or NACK signal should be transmitted from the receiver to the transmitter. Usually, one bit signal is used for the ACK signal or the NACK signal. In other word, the transmitter recognizes that the transmitted packet is received by the receiver in correct when the receiver transmits an ACK signal of one bit (for example 1 (one)), and recognizes that the packet is not received correctly by the receiver and retransmits a corresponding data packet when the receiver transmits a one bit NACK signal (for example −1 (minus one)).
A system using a High Speed Downlink Packet Access (hereinafter referred to HSDPA) of UMTS of an IMT-2000 standard of Europe defines that user equipment transmits an uplink ACK/NACK signal of 1 (one) bit for a downlink data packet transmission of a base station (Node B).
In a mobile radio communication system that uses packet type data transmission scheme such as above system, the ACK/NACK signal transmitted by the receiver is designed to be transmitted in a high power and energy without any specific protection such as channel coding for the purpose of rapid recognition of the acknowledgement. For example, the HSDPA system that is presently under standardization defines that the user equipment should transmit 1 (one) bit ACK/NACK signal without channel coding through the uplink so that the transmitter may be informed whether the corresponding data packet was received or not.
FIG. 1 illustrates how the background art receiver utilizes the one bit to transmit the response signals, which are complementary. The threshold power level is moved to the direction of ACK determination region from 0- at the transmitter which receives the acknowledgment signal from the receiver, for the transmitter to determine (DTX: discontinuous transmission, that is, no acknowledgement is transmitted) as NACK. However, in the receiver, the actual transmission power for the NACK signal is substantially same as the actually transmission power for the ACK signal in absolute power level (ΔA).
Since the receiver (for example, the user equipment using High Speed Downlink Packet Access scheme (hereinafter referred to as HSDPA)) uses one bit to transmit the ACK/NACK signal, the actual transmission power level for the one bit is intentionally set at high power and energy for the purpose of rapid recognition of the data packet acknowledgment.
In this particular case, the actual transmission power of the ACK signal is substantially equal to the required power such that there is a ΔA power difference relative to 0. Similarly, the actual transmission power of the NACK signal is substantially equal in opposite polarity with the ΔA power difference relative to 0. The power level is a relative value from the reference value of the radio channel floating according to the channel condition.
Though the transmitter transmits a data packet to the receiver, such a case may occur that the receiver does not send any acknowledgement (ACK/NACK signal) because it did not detect the existence of data packet to be received.
In this case (DTX: Discontinuous Transmission), if a system is designed to assume that the data packet is received or not within a specific time period after the transmitter transmits the data packet and the receiver did not send any acknowledgement (ACK or NACK signal) within the time period, and if the threshold power level is 0 at the transmitter, the transmitter is forced to determine the absence of acknowledgement to be one of the ACK signal or the NACK signal.
If no response (DTX) is determined as the ACK signal, the transmitter considers that the receiver has received the corresponding data packet in correct and transmits the next data packet, so that there occurs a data loss. To prevent the data loss, the transmitter should consider the DTX case as the case that the NACK signal is received. To achieve this, a threshold for determining whether the ACK/NACK signals are received may be shifted to a polarity of the ACK signal, as shown in FIG. 1.
In a fading radio channel circumstance of a radio communication system, the possibility of determining error of the acknowledgement signal such that the ACK signal transmitted from the receiver is determined as the NACK signal (ACK_NACK error) or the NACK signal is determined as the ACK signal (NACK_ACK error) may be greater than that of a wired channel circumstance of a wire-line communication system. The effects of the two types of errors to the system performance are not same to each other. So, the acceptable possibilities of the determining error are being defined in the course of standardization of HSDPA. Unfortunately, the background system inefficiently uses the same transmitting power for both the ACK signal and the NACK signal.